A conventional turbofan engines is require to generate electrical power for ancillary systems both in the engine and the associated installation. Such conventional arrangements utilise an accessory gearbox that takes drive from the turbofan main shaft(s) and drives a separate electrical generator.
FIG. 1 illustrates a two-shaft gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows A and B. The gas turbine engine 10 comprises a core engine 11 having, in axial flow A, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines, in axial flow B, a bypass duct 22 and a bypass exhaust nozzle 18. The fan 23 is attached to and driven by the low pressure turbine 19 via shaft 26 and epicyclic gearbox 30.
The gas turbine engine 10 works in a conventional manner with air in the core airflow A being accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted.
The resultant hot combustion products then expand through, and thereby drive the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.
A known mechanical arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith, in a conventional manner, is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.
The epicyclic gearbox 30 is of the planetary type, in that the planet carrier 34 rotates about the sun gear 28 and is coupled to an output shaft via linkages 36. In other applications the gearbox 30 may be a differential gearbox in which the ring gear 38 also rotates in the opposite sense and is coupled to a different output shaft via linkages 40. An epicyclic gearbox 30 must be lubricated, by oil or another fluid. However, the oil becomes heated by being worked during operation of the epicyclic gearbox 30.
A geared turbofan engine utilises a fan reduction gearbox to provide rotational drive to the fan at a lower speed that the gas turbine core engine. The use of a second gearbox (as outlined above) to drive an electrical generator adds weight and complexity.